To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like.
In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
With rapid development of information industry, especially increasing requirements from mobile Internet and Internet of things (IoT), mobile communication techniques are facing unprecedented challenges. According to International Telecommunication Union (ITU) report ITU-R M.[IMT.BEYOND 2020.TRAFFIC], it can be predicted that as of 2020, mobile service amount will increase 1000 times compared with 2010 (4G era), and the connected user devices will exceed 17 billion. With involvement of IoT devices into the mobile communication networks, the number of connected user devices may be more astonishing. Under the unprecedented challenges, communication industry and the academia have started intensive researches in fifth generation mobile communication techniques (5G) facing 2020. At present, architecture and global objective of future 5G have been discussed in the ITU report ITU-R M.[IMT.VISION], which provides detailed description to requirement prospect, application scenarios and various important performances of 5G. With respect to new requirement of 5G, the ITU report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] provides information related to technology trends of 5G, aims to solve dramatic problems such as system throughput, user experience consistency, extendibility, supporting IoT, tendency, efficient, cost, network flexibility, supporting of new services and flexible spectrum utilization.
Modulation waveform and multiplexing manner are important basis for air-interface design of radio communications, and there is no exception for 5G. At present, a typical representation Orthogonal Frequency Division Multiplexing (OFDM) of multi-carrier modulation (MCM) techniques has been widely used in broadcast audio and video fields and civil communication systems, e.g., Long Term Evolution (LTE®) system corresponding to the Evolved Universal Terrestrial Radio Access (E-UTRA®) protocol defined by 3rd Generation Partnership Project (3GPP®), European digital video broadcasting (DVB) and Digital Audio Broadcasting (DAB), Very-high-bit-rate Digital Subscriber Loop (VDSL), IEEE 802.11a/g Wireless Local Area (WLAN)®, IEEE 802.22 Wireless Regional Area Network (WRAN) and IEEE 802.16 world interoperability for microwave access (WIMAX®), etc. It is well-known that, basic idea of OFDM technique is to divide wideband channel into multiple parallel narrow-band sub-channels/subcarriers, such that high rate data stream transmitted in frequency selective channel becomes low rate data streams transmitted on multiple parallel flat sub-channels, which improves anti-multipath interference ability of the system, and OFDM may simplify implementation of modulation and de-modulation via IFFT/FFT. Further, the addition of a Cyclic Prefix (CP) changes a cyclic convolution into a circle convolution. According to performance of the circle convolution, if the CP length is larger than the maximum channel multipath delay, Inter-Symbol Interference (ISI) may be avoided through simple single-tap frequency domain equalization. Thus processing complexity of the receiver is decreased. Although modulated waveform based on CP-OFDM is able to meet service requirement of Mobile Broadband (MBB) well, 5G faces more challenges and more various scenarios, CP-OFDM has much limitations and shortcomings in 5G scenarios which mainly include the following.
(1) In a low delay transmission scenario of 5G, the adding of the CP to resist ISI greatly decreases spectrum utilization ratio, since the low delay transmission extremely shortens the length of OFDM symbol and the length of the CP is merely subject to the channel impulse response, compared with the length of the OFDM symbol, the length of the CP greatly increases. Such overhead leads to high spectrum efficiency loss and is hard to be accepted.
(2) In an IoT scenario of 5G, rigid time synchronization requirement results in large signaling overhead for maintaining close-loop synchronization. And the rigid synchronization scheme makes the frame structure inflexible, which cannot support different synchronization requirements of different services.
(3) OFDM has a large out-of-band leakage due to the utilization of rectangular pulse, since such waveform makes side lobe attenuates very slow, which is also the reason why OFDM is very sensitive to central frequency offset. However, 5G may have many fragmented spectrum flexible access/sharing requirements, the out-of-band leakage of OFDM greatly restricts the flexibility of spectrum access, i.e., requires a wide frequency-domain guard band, thus the spectrum utilization ratio is decreased.
These defects are caused by its inherent characteristics. Although actions may be taken to reduce impacts caused by the defects, system design complexity may be increased and the problem cannot be solved essentially.
Due to the above, as stated in ITU report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS], some new waveform modulation techniques (based on multi-carrier modulation) are in consideration of 5G. Among them, Filter Bank Multiple Carrier (FBMC) modulation technique is a hotspot. It provides freedom for the design of prototype filter and may adopt filters with better Time/Frequency Localization (TFL) to perform impulse forming to the transmitted waveform, such that the transmitted signal may exhibit various excellent features including: not requiring CP to resist ISI which improves spectrum efficiency, low out-of-band leakage which supports flexible fragmented spectrum access better, and non-sensitive to frequency offset. Usually, a typical FBMC system adopts an Offset Quadrature Amplitude Modulation (OQAM) technique to maximize the spectrum efficiency, generally referred to as FBMC/OQAM system, or OFDM/OQAM system. Reference may be made to a prior document “Analysis and design of OFDM/OQAM systems based on filter bank theory”, IEEE Transactions on Signal Processing, Vol. 50, No. 5, 2002 for applications of FBMC in digital communications.
FBMC has some advantages that OFDM does not have and therefore receives much attention in 5G researches. But some inherent defects of it bring challenges to its application in radio communication system. The challenges need to be solved are researched continuously. One dramatic problem is that the filter adopted by FBMC results in a long tail effect to time domain waveform, also referred to transition period problem. During uplink transmission based on short data blocks, if the length of the data blocks is extended to include the tail in order to avoid overlapping of the tail with other data blocks, the number of symbols transmitted in valid time period is reduced, which greatly decreases spectrum efficiency. Therefore there is an idea that FBMC is merely applicable for long data transmission. On the contrary, if the length of the data block does not include the tail, which means the tail will overlap with other data blocks which may cause large interference if not better processed and thereby also restricting the spectrum efficiency. A present method cuts off the tail, so as to avoid overlapping of the tail with other data blocks. But the cutoff of the waveform leads to signal distortion which also affects the spectrum efficiency. In addition, the cut-off signal has a spread spectrum which increases Inter-Carrier-Interference (ICI). Therefore, such cutoff method is not effective.
In view of the above, in order to make FBMC more competitive in 5G candidate techniques, besides utilizing and developing its advantages, we also need to overcome its shortcomings. With respect to the service mode of sporadic access in 5G especially IoT scenarios, it is necessary to find an effective method to solve the tail effect of FBMC.